Python 3.10 support

[mgedmin]

cpython git master calls itself Python 3.10.0a0, so it's time to consider Python 3.10 support.

There are places where I sort version numbers alphabetically, which puts 3.10 before 3.5. For an example check out https://github.com/jmoiron/humanize:

mg@blynas: /tmp/humanize [git:master=] $ check-python-versions 
setup.py says:              3.5, 3.6, 3.7, 3.8, PyPy
- python_requires says:     3.5, 3.6, 3.7, 3.8
tox.ini says:               3.5, 3.6, 3.7, 3.8, PyPy3
.travis.yml says:           3.10-dev, 3.5, 3.6, 3.7, 3.8, 3.9-dev, PyPy3

[mgedmin]

Actually manylinux code has a regex that assumes two digits for the version number (cp39 etc), and that needs to change.

As soon as I figure out if the 3.10 tag will be cp310 or cp3_10, see pypa/wheel#354

[hugovk]

As soon as I figure out if the 3.10 tag will be cp310 or cp3_10, see pypa/wheel#354

The 3.10 tag is cp310: pypa/packaging#355

[mgedmin]

Yay, cp310 is what the current codebase assumes, so all I have left to do is bump the CURRENT_PYTHON_3_VERSION constant in versions.py and I'm done!

[mgedmin]

Oh wait, I see pip downloads coverage 6.0.1, and then 5.5, and then installs 5.5.

Is it because I install coveralls, which requires coverage < 6.0 or something??

[mgedmin]

Ok, reproduced this locally with Python 3.10 and coverage 5.5.

This could be nedbat/coveragepy#1205, fixed in coverage 6.0. Unfortunately coveralls doesn't support coverage 6.0: TheKevJames/coveralls-python#326.